I have just been watching the women's team sprint finals, and saw China lose the final race due to a rule infringement. Earlier the Great British team were disqualified for the same reason. There is a set region that one rider has to peel off and let the other pass through.
Despite the commentators' best efforts I still don't understand where this region is marked on the course.
Also I don't understand why there is such a rule at all. The lead rider allows the second rider to draft for the start of the race, so they want to stay in the race as long as they can, but they wouldn't want to stay in until the end. Is there a reason for a fixed change-over zone (e.g. safety), or is it just a rule that's always been part of the sport?